The invention is concerned with rotary boring of earth. More precisely, the invention relates to a cutter assembly for rotary boring of earth, comprising a boring head, at least one rotating cutter having a shaft the end portions of which protrude from the ends of the cutter; a mounting member arranged, i.e., secured, in the boring head for the rotating cutter and provided with two mutually spaced legs the free ends of which are provided with curved counter surfaces for receiving the ends of the shaft of the rotating cutter; and fastening elements provided with a curved counter surfaces, the fastening elements being arranged at the ends of the legs of the mounting member for fastening the shaft of the rotating cutter to the mounting member. The invention further relates to an arrangement for fastening a rotating cutter to a boring head.
Assemblies of this kind are today well-known in connection with the boring of large-diameter holes, such as raises and other such tunnels. The boring of raises and the like is principally carried out, for instance, in the following way. A small diameter hole called a pilot hole is first bored from above downwards, for instance. After the boring of the pilot hole has advanced to a lower level, a large-diameter boring head is attached to the drill rod. After the boring head is positioned in place, it is rotated and simultaneously drawn upwards e.g. hydraulically in the direction of the pilot hole. Due to the rotation of the boring head while it is drawn upwards, the rotating cutters in the boring head break the earth so that a large-diameter raise is obtained.
Due to this principle of operation the structure of the cutters of the boring head is of particularly great importance. The cutters and their fastenings have to withstand very great shock stresses, compression stresses, axial stresses and torsional stresses. Furthermore, the structure has to be tight in order to protect the journalling against impurities. Besides a steady structure, the fastening of the cutters has to be such that the cutters can be detached by means of conventional tools.
Various solutions for the fastening of cutters are known in the art. Known solutions include e.g. the solutions disclosed in U.S. Pat. Nos. 3,612,196, 3,705,635, 3,863,994, 4,241,799 and 4,448,271.
Sometimes known solutions are complicated and/or the fastening is sensitive to wear, i.e. the fastening does not compensate for a clearance caused by wear, so that the service life of the device is short. This is due to the fact that the clearances caused by wear greatly accelerate the wearing process.
The object of the invention is to provide a cutter assembly and an arrangement for the fastening of the cutters, by means of which the disadvantages of the prior art can be eliminated. This is achieved by means of a cutter assembly and a fastening arrangement according to the invention, which are characterized in that the curvature of the middle portion of the curved counter surface provided on the fastening element around the end portion of the shaft of the rotating counter being so formed that a gap narrowing in the direction of the periphery of the shaft is defined between the end of the shaft and the middle portion of the counter surface of the fastening element and the assembly further comprising a rigid clutching part provided at the end of the shaft and extending outside the outer periphery of the shaft in the radial direction, the clutching part being pressed against the counter surface of the fastening element when the shaft turns, thus preventing the turning movement of the shaft in this direction.
An advantage of the invention is its simplicity, which makes the manufacture of the devices according to the invention inexpensive. The operation of the invention is also very advantageous, since the cutters will not get stuck as in some prior solutions. The cutters can be replaced with conventional tools. The solution according to the invention prevents the rotations of the shaft irrespective of the direction of rotation of the boring head. The solution according to the invention also automatically compensates any clearances caused by wear, so that the wear does not cause early failure of the attachment mechanisms.